Diesel Hybrids: Back to the Future?

by Henry Kelly

Inherently more efficient than the Otto cycle engines that power most U.S. cars, diesel engines dominate markets for trucking, construction equipment, pipeline pumps and many other applications. But they have a well-deserved reputation for being foul smelling polluters–not something that you’d want to invite home. Their reputation in U.S. markets was further sullied during the 1980s when General Motors introduced a diesel car that has become a legendary disaster on the scale of Ford’s Edsel fiasco. Today diesels are less than 1 per cent of the U.S. personal vehicle fleet.

In Europe, diesel’s market share of car sales jumped from 20 per cent to 40 per cent in the past decade because of its fuel efficiency and superior performance, particularly high torque at low RPM. The major barrier to expanding markets for diesels is their reputation for being polluters. Few of the models sold in Europe meet present U.S. emission standards and they are even farther from meeting our stricter standards just coming into force.

But this situation has been transformed by dramatic changes in technology and regulations that have gone almost unnoticed. Given the right incentives, new generations of highly efficient diesel engines, diesel hybrids, and dramatically cleaner diesel fuels can provide a practical, affordable way to provide Americans the transportation they want while slashing pollution and fuel use. And these changes could be made in the next few years, since diesels require comparatively modest changes in US vehicle manufacturing and fuel infrastructures.

While it is possible that fuel cells may eventually be less expensive and more efficient than diesels, the research challenges remain enormous and the risks high. Indeed it may take a decade or more to develop a commercial fuel cell system that even matches today’s diesel performance – let alone cost. The main risk with today’s diesels is not whether they can achieve efficiency and emission goals but whether the cost of meeting these goals will be acceptable to markets in the next few years. At the very least, diesel vehicles will set a high standard for any other new technology to meet. It makes no sense to miss the benefits of a proven technology to wait and hope for fuel cell breakthroughs.

Exact comparisons between different fuel and power systems are difficult since it is important not just to compare peak efficiencies but the average efficiency of an engine in a realistic driving cycle. Mile per gallon comparisons must also be adjusted to reflect the different energy content of fuels–diesel fuel contains about 13 per cent more energy per gallon than gasoline. Also, different amounts of energy are required to manufacture the fuel itself.

The accompanying chart compares the energy efficiency of different fuel/vehicle systems. The top pair of bars shows gasoline engines today; the next pair represent today’s diesel systems. The middle pair of bars show results for a diesel/hybrid prototype. The three lower pairs of bars show fuel cell vehicles planned for the future, including targets DOE has set for a hypothetical fuel cell vehicle in 2010. The performance ratings of the diesel/hydraulic hybrid prototype vehicle clearly steal the show, delivering a huge gain in overall energy that actually propels the vehicle – 28 per cent compared to today’s gasoline engines, which direct just 14 per cent of the energy used “from well to wheels”–to turning the wheels. That’s a 100 per cent improvement.

It’s important to recognize that hydrogen fuel cell manufacturers face a nasty series of engineering challenges even to produce the near-term fuel cell systems shown on the chart that meet U.S. consumer expectations for price, reliability, range and lifetimes.

The final pair of bars reflects the DOE 2010 goal of a fuel cell capable of using a hydrocarbon fuel.1 There is no theoretical reason why these goals can not be achieved and research to pursue them should clearly continue. But the goals are astonishingly bold given the current state of the art in fuel cell and fuel reforming cost and performance. What is striking, however, is that even if the ambitious, high-risk goals are met, the systems would achieve fuel efficiencies that are only 18 percent higher than the diesel hybrid system already in prototype.

Three Transforming Changes

Three critical factors in the United States are transforming the diesel as we know it: fuel standards, incoming “Tier 2” EPA vehicle emissions standards, and engine technology.

For decades, diesel fuels were allowed to be dirtier than gasoline, but this is changing. Refiners were allowed to sell diesel fuel having up to 500 parts per million (ppm) of sulfur; sulfur at these levels effectively destroys catalytic converters used to lower other emissions. But the new fuel standards, issued at the beginning of the current administration, require the allowed maximum sulfur content in diesel fuel to drop by a whopping 97 per cent to 15 ppm, by 2009. Refiners expect average fuel to contain less than 15 ppm sulfur and several are selling low sulfur fuels now in preparation for the final phase-in which begins in 2006.2 (By that year the sulfur content of gasoline fuel will also change, going from about 200 ppm today to an average of 30 ppm by 2006.3)

This striking reduction in the sulfur content of diesel fuels made it possible for EPA to set extremely strict limits on other diesel emissions. So under Tier 2 standards, diesel vehicle emissions will finally catch up–that is, go down to–the standards for NOx and particulates of gasoline-fueled cars and light trucks. These must average 0.07 grams per mile (gpm) of NOx by 2009–an 80 per cent reduction from the 0.3 gpm allowed earlier. Diesels, playing catch up, will have to reduce NOx by 93 per cent by 2009 from the 1gpm previously allowed for the larger personal vehicles. The new emissions standards also drop particulate levels to roughly 0.01 gpm. Vehicle manufacturers will be allowed to average diesel and gasoline vehicles to meet standards. There is no ironclad guarantee that diesel vehicles won’t be slightly above the average but no vehicle in mass production could stray far from the average.

As for heavy trucks, which are mostly diesel-fueled, the new standards require them to emit no more than 0.2 grams per horsepower-hour of NOx and 0.01 gram of particulate per horsepower-hour by 2010, with phase-in beginning in 2007. EPA argues that new technology will allow the standards to be met with little or no fuel economy penalty.

Besides fuels and emissions, there is technology. Thirty manufacturers surveyed by EPA confirmed they can meet the new standards by advances in particulate traps, emission controls, and other devices. (There is some question whether the efficiency of diesel engines might be somewhat reduced.) On the other hand, powerful, low-cost computers can now control how fuel and air are injected into diesel cylinders so that temperatures inside stay below the 2100 degrees K where NOx is formed– almost eliminating NOx emissions.

The white bars show the efficiency of the vehicle; the black bars show overall efficiency of the fuel and its vehicle for various fuel/vehicle types. The overall efficiency of the diesel / hybrid system (third black bar from top) is greater than today's hydrogen fuel cell systems because it is far more efficient to produce diesel fuel from oil than it is to produce hydrogen fuel from natural gas, the most likely source. Even the highly efficient fuel cell vehicle proposed by the DOE for 2010 (white bar at bottom) ends up being just 18 percent more efficient (black bar at bottom) overall than current diesel / hybrid prototype.

Sources: Industry, academic and government studies.

Diesel Hybrids

The new diesel engines are well suited for use with hybrids, such as a hydraulic/diesel setup being explored by International, Ford and others. Hydraulic hybrids use compressed nitrogen instead of batteries to store energy. When the brakes are pressed, a much higher fraction of the energy captured is returned to the wheels. If a hydraulic motor could power the vehicle, it would be possible to integrate the engine and pump in a free-piston system that could be simple and inexpensive.

The point is that these technologies are available now or within the next five years–in sharp contrast to more fuel cell technology that faces large technical uncertainties. But what would diesels cost and where would we get more diesel fuel?

Advanced diesel technology could increase the fuel efficiency of a large 4-wheel drive SUV from 17.2 mpg in a baseline vehicle to 23.6 mpg, for an incremental cost of about $1,600. Use of a hydraulic hybrid could achieve 32 mpg for an incremental cost of $2,200. Including improved tires and aerodynamic designs, the vehicles could achieve 38.2 mpg for an incremental cost of $2,500. These estimates are not in the distant yonder, but come from a detailed study published by EPA in January 2004, which assumed high volume production.4

One of the benefits of hydrogen is that it can be produced from many sources. But diesel fuels also can be produced from a variety of fuel resources. Since diesel is a liquid fuel, expanding diesel’s use does not require replacing current filling stations and adding new infrastructure, as expanded use of hydrogen would require.

Diesel fuels do not require the high quality petroleum or extensive refining required to make gasoline. Fuels that work in diesel engines can be made from otherwise undesirable heavy oils available in many parts of the world including the U.S. and Canada. Diesel systems can run on dimethyl ether or methanol made from natural gas. Given the right economics it’s also possible to make diesel fuel from coal. All of these resources, of course, are carbon-based. The way diesel systems would help reduce greenhouse gas emissions is due to their much higher efficiency.

Even greater reductions in greenhouse emissions can be achieved given technologies to make diesel fuels from organic waste or biomass crops.

The bottom line is that diesel technology has been advancing steadily for the past decade–even while the public debate about the future of cars has gotten sidetracked on a possibly quixotic quest for fuel cells. Yet humble diesel could increase the fuel economy of new vehicles entering the U.S. fleet by 25 to 100 per cent (in hybrids) and at affordable prices. If the whole fleet were diesel, it could use one fourth less of the total fuel used today–the equivalent of cutting all the oil we get now from Saudi Arabia.

Where is the public debate going? Environmentalists are paranoid that the new emission standards will be rolled back, and some lobbyists are trying to do just that. For instance Rep. Mac Collins (R-Ga.), who owns a family trucking business,5 asked the GAO for a study of the standards’ impact –probably expecting the answer to be dire. But the GAO found no such thing; in fact it found reassurance from industry, no less, that the standards can be met. The report said:

“Representatives of the association of emissions control technology manufacturers and the five engine manufacturers we contacted said that the technologies to control diesel emissions have advanced. While they acknowledged that several technical problems remain, all of the engine manufacturers reported that they expect to have engines ready by 2007 and plan to have prototype engines ready for trucking companies to test by mid- to late 2005.” 6 In other words, they confirm the conclusions of the recent EPA review, namely that makers of diesel heavy-duty trucks are ready to meet the standards.

Demand for diesel would grow if Congress required fuel economy of at least 36 mpg, or if it passed a fuel tax that reflected the real costs of the gasoline fleet’s dependence on high-grade foreign oil. But such sensible policies are unlikely, to say the least. Meanwhile, modest tax credits to consumers and to manufacturers, coupled with strict adherence to the present standards, could start getting diesels on track for a bigger role. It is an opportunity too great to ignore.

Henry Kelly is the President of the Federation of American Scientists.